Sin for me is once you commit a mistake or a things that is against to devine law or the 10 commandments of God.  Even if it is visible or not as long as it is not right on the eyes of people or God it will always a sin.

Other information.

In a religious context, sin is the act of transgression against divine law. Sin can also be viewed as any thought or action that endangers the ideal relationship between an individual and God; or as any diversion from the perceived ideal order for human living. "To sin" has been defined from a Greek concordance as "to miss the mark"